Left: phase space diagram of the injection of He⁺ pickup ions for the most likely configuration of the solar magnetic field vector, i.e. φ_B = 45° ± 15°. For this configuration, most He⁺ ions are injected in the sunward hemisphere of phase space (w < 1.0) and will not be detected by an instrument like SWICS, CTOF, or PLASTIC. Right: during almost perpendicular configurations of B, the He⁺ ring beam (red line) lies within the instrument aperture at an angle of incidence of ~ 0°. However, He+ ions that have been ionised prior to the perpendicular configuration of B, i.e. the phase space density shown in the left panel, are now also scattered into the anti-sunward hemisphere of phase space. This scenario causes a distinct one-sided asymmetry of the observed He⁺ ring beam distributions during perpendicular configurations of B.